The invention relates to a method for monitoring a nitrogen oxide storage catalytic converter (NSC) in the exhaust gas duct of an internal combustion engine which is operated at least temporarily in a lean fashion, wherein during a lean mode of the internal combustion engine nitrogen oxides from the exhaust gas are stored by the nitrogen oxide storage catalytic converter, wherein during a regeneration phase of the nitrogen oxide storage catalytic converter the internal combustion engine is operated in a rich fashion, and as a result the nitrogen oxides stored in the nitrogen oxide storage catalytic converter are removed, and wherein an exhaust gas component or exhaust gas characteristic variable which is characteristic of the profile of the regeneration is detected during the regeneration phase by means of an exhaust gas probe.
The invention also relates to a device, in particular a diagnostic unit, for carrying out the method according to the invention.
In order to reduce fuel consumption, what are referred to as lean engines have been developed in the field of petrol engines, said lean engines being operated with a lean air/fuel mixture in the partial load mode. This mixture contains a comparatively higher oxygen concentration than is required for the complete combustion of the fuel. The oxidizing components, such as oxygen (O2) and/or nitrogen oxides (NOx), are then present in excess in the exhaust gas compared to the reduced exhaust gas components such as carbon monoxide (CO), hydrogen (H2) and/or hydrocarbons (H2). In contrast, diesel engines generally operate under operating conditions with widely superstoichiometric air/fuel mixtures.
In order to be able to reduce the nitrogen oxide emissions of internal combustion engines which are operated in a lean fashion it is known to provide NOx storage catalytic converters (Nitrogen Oxygen Storage Catalyst—NSC) in the exhaust gas ducts. An NOx storage catalytic converter stores nitrogen oxides from the exhaust gas, while the internal combustion engine is operated with an excess of air, that is to say a superstoichiometric air/fuel ratio with a lambda value >1.
In order to maintain the storage capability of the NOx storage catalytic converter, the stored nitrogen oxide has to be removed from time to time. In order to carry out such a regeneration of the NOx storage catalytic converter it is known to establish a reducing exhaust gas atmosphere in which the stored nitrogen oxide is converted into nitrogen. For this purpose, the internal combustion engine is operated in a rich fashion that is to say with a substoichiometric air/fuel ratio with a lambda value <1. After such regeneration, the NSC can absorb nitrogen oxides again.
The currently valid regulations for on-board diagnosis (OBD) in Europe and in the USA require nitrogen oxide storage catalytic converters to be monitored in terms of their emission-reducing effect on nitrogen oxides (NOx).
EP 1 831 509 B1 discloses a method for monitoring the nitrogen oxide storage capability of a nitrogen oxide storage catalytic converter, used as a starting catalytic converter, in an exhaust gas purification system of a motor vehicle with a lean engine, which exhaust gas purification system contains a catalytic converter system composed of the starting catalytic converter and a main catalytic converter which is also embodied as a nitrogen oxide storage catalytic converter. In this context, in a normal operating mode of the catalytic converter system, when the evaluation of nitrogen oxide slip downstream of the main catalytic converter system exceeds a regeneration criterion, in each case total regeneration of the catalytic converter system is performed by briefly switching over the engine from lean mode to rich mode. In this context it is provided that in order to check the nitrogen oxide storage capability of the starting catalytic converter the entire catalytic converter system is loaded with nitrogen oxides until the regeneration criterion is reached, and partial regeneration, which comprises regeneration of only the starting catalytic converter, of the catalytic converter system is performed. For this purpose, rich mode is ended and the system is switched back into the lean mode if a breakthrough of rich exhaust gas is registered between the starting catalytic converter and the main catalytic converter, and for which purpose the time period after the switching back into the lean mode until the regeneration criterion is exceeded again is measured downstream of the catalytic converter system as a measure of the nitrogen oxide storage capability of the starting catalytic converter.
A robust OBD means here, in particular, that a monitoring function has to be made available which can differentiate an intact NSC, referred to as a WPA (“worst part acceptable”) model in terms of legislation, from a defective NSC, referred to as a BPU (“best part unacceptable”) model in terms of legislation.
Damage to the NSC brings about, for example, a decrease in the consumption of reducing agent during a complete regeneration. This variable is accessible by means of two lambda probes, which are installed upstream and downstream of the NSC, with the result that it can be used as a monitoring feature for diagnosis of the NSC. In particular precise determination of the lambda values by means of the two exhaust gas probes are essential in terms of the robustness and sensitivity of the method for monitoring the consumption of reducing agent.
In order to achieve better separation between the BPU (“best part unacceptable”) and WPA (“worst part acceptable”), in particular plausibility-checking functions are carried out only under certain monitoring conditions.
In the field of exhaust gas post-treatment, monitoring operations for this purpose are frequently limited to specific value ranges for one or more of the following variables, on a modeled or measured basis. These variables may be, for example, the exhaust gas mass flow, the exhaust gas volume, the exhaust gas temperature at any desired location, the operating point (rotational speed, injection quantity), the vehicle speed, the ambient pressure, the ambient temperature, the signal values for, for example, the oxygen content, nitrogen oxide content, hydrocarbon content or carbon monoxide content, the exhaust gas recirculation rate (EGR), the engine operating mode, the engine status, the engine running time and/or the engine service life.
Furthermore, for the same reason, monitoring operations are frequently carried out under (quasi-) steady-state conditions which are determined on the basis of one or more of the abovementioned variables.